Acquired immunodeficiency syndrome, or AIDS, is a fatal disease which has reached epidemic proportions among certain high risk groups. Several features of AIDS make therapy extremely difficult. The main target of the AIDS virus, now known as HIV, or human immunodeficiency virus, is the T4 lymphocyte, a white blood cell that marshals immunodefenses. The virus depletes T4 cells. This depletion in AIDS causes a severe depression of the immune response. Thus, to be effective against AIDS any drug must modify virus effect without much help from host immunity. Furthermore, the virus also affects cells in the central nervous system, where it is protected by the blood-brain barrier from compounds that might otherwise be effective against the virus. In infecting its host, the HIV binds to specific cell-surface receptor molecules. The virus penetrates the cell cytoplasm and sheds its protein coat, thereby releasing its genetic material, a single strand of RNA. A viral enzyme, reverse transcriptase, accompanies the RNA. The virus, a retrovirus, thereby transcribes the RNA into DNA. Ultimately, some DNA copies of the HIV genome become integrated into the chromosomes of the host cell.
The integrated viral genome, known as a provirus, may remain latent until the host cell is stimulated, such as by another infection. The proviral DNA is then transcribed into mRNA, which directs the synthesis of viral proteins. The provirus also gives rise to other RNA copies that will serve as the genetic material of viral progeny. The proteins and the genomic RNA congregate at the cell membrane and assemble to form new HIV particles, which then break off from the cell. Two HIV genes, tat and trs/art, appear to control this burst of replication, which destroys the cell. These genes code for small proteins that boost the transcription of proviral DNA and the synthesis of viral proteins.
Several compounds have been shown to reduce the activity of reverse transcriptase in vitro. Reverse transcription is a step that is essential to viral replication but irrelevant to host cells. It has been found that HIV replication is considerably slower in the presence of compounds such as suramin, antimoniotungstate, phosphonoformate, and a class of nucleoside analogues known as dideoxynucleosides.
Nucleoside analogues are a class of synthetic compounds that resemble the naturally occurring nucleosides, which are chemical precursors of DNA and RNA. A nucleoside comprises a single or double-ring base linked to a five-carbon sugar molecule. An analogue differs from the naturally-occurring nucleoside in large or small features of the base or the sugar. An enzyme that normally acts on a nucleoside in the course of viral replication may also bind to a nucleoside analogue. Because the naturally occuring nucleosides and nucleoside analogues differ, however, binding to the analogue can incapacitate the enzyme and disrupt a molecular process crucial to vital replication.
Of the synthetic nucleoside analogues, dideoxyadenosine (ddA) has been found to have potent in vitro activity against the human immunodeficiency virus which causes AIDS. The activated form of the dideoxynucleosides, their 5'-triphosphates, appear to inhibit replication of the virus at the stage of reverse transcription of de novo infection of the virus. Due to this, it is most likely that a drug of this type must be taken continuously if its therapeutic effect is to be maintained. Since daily treatments may extend over long periods of time, oral drug administration is envisioned as the most practical route for patient populations.
Drugs administered orally are exposed to pH ranges of 1 to 2 in the human stomach environment for approximately one hour. In the case of ddA this may provide for drug instability, since this compound undergoes acid-catalyzed hydrolysis of its glycosidic bond at a rate 40,000 times faster than adenosine. ddA has a t.sub.1/2 of 35 seconds at a pH=1.0 at 37.degree. C. Cleavage of this compound thus not only reduces its efficacy, but potential problems of toxicity may occur due to formation of excessive quantities of one of its cleavage products.
2'-F-substituted dideoxynucleoside derivatives of dideoxyadenosine (ddA) were originally disclosed in 1987 (Marquez et al, Biochem. Pharmacol. 36(17):2719-2722 (1987); U.S. patent application Ser. Nos. 07/039,402, filed Apr. 17, 1987 and 07/288,652, filed Dec. 12, 1988). Among the disclosed compounds are 6-amino-(.beta.,D-2',3',dideoxy-2'-fluororibofuranosyl)-9-H-purine or 2'-F(ddA) and 6-amino-9-(.beta.-D-2',3'-dideox-2'-fluoroarabinofuransyl)-9H-purine or 2'-F-ara-ddA.
The first compound was obtained from 3'-deoxy-ara-A by a four-step process involving protecting the 5'-hydroxyl group with dimethoxytrityl chloride, activating the 2'-hydroxyl group by forming the corresponding triflate, inverting the configuration at the 2'-position by a Sn.sub.2 displacement using tetra-n-butyl ammonium fluoride, and removing the dimethoxytrityl protective group using dichloroacetic acid.
The second compound was prepared by condensing 6-chloropurine with 3-1-acetyl-5-O-benzoyl-2-deoxy-2-fluoro-D-arabinofuranosyl bromide, separating the expected four isomers produced, characterizing the correct 6-chloroisomer, and subjecting this isomer to ammonolysis with concentrated methanolic ammonia to give 6-amino-9-(.beta.-D-2'deoxy-2'-fluoroarabinofuranosyl)-9H-purine or 2'-F-ara-dA. The selective protection of the 5'-hydroxyl function of this compound with t-butyl dimethylsilyl chloride yielded a product permitting a 2-step reduction of the 3'-hydroxy group. Further treatement with phenylchlorothionocarbonate, followed by reduction of the intermediate 3'-O-phenoxythiocarbonyl derivative with tri-n-butyltin hydride produced the desired 2',3'-dideoxy nucleoside. The 5'-blocking group was then removed with tetra-n-butyl ammonium fluoride to yield 2'-F-ara-ddA.
The latter compound has the fluorine stereochemically placed in the .beta. configuration at the 2'-position, or up, and is as potent an anti-HIV compounds as ddA in the HIV/ATH8 test system. The earlier compound has the fluoride at the 2'-position in the .alpha. configuration, or down, however, and it affords only about 13% of the protection that ddA exhibits against HIV. It also is more toxic than ddA.
U.S. Pat. No. 4,625,020 to Brundidge et al discloses a method of producing 1-halo-2-deoxy-2-fluoroarabinofuranosyl derivatives bearing protective ester groups from 1,3,5-tri-O-acyl-ribofuranose. The 1-halo derivatives are intermediates in the synthesis of therapeutically active nucleosidic compounds.
EP Laid Open Application No. 010,205 discloses 5-substitued 1-(2'-deoxy-2'-substituted-.beta.-D-arabinofuranosyl) pyrimidine nucleosides where the 2'-substituent is halogen, alkylsulfonyl or arylsulfonyl.
U.S. Pat. No. 4,908,440 to Sterzycki et al discloses other 2'-3'-dideoxy-2'-fluoronucleosides and 2',3'-dideoxy-2',3'-didehydro-2'-fluoroneucleosides which are useful for anti-HIV therapy and a method for their preparation. The synthesis scheme shown on columns 1 and 2, Scheme I, bears some similarity to that of the invention. However, in this scheme no fluorine atom is on the ring and there is no obvious way of making the 2'-fluoro (alpha or beta) derivative of the compound 1 of this scheme.